This invention relates generally to the injection moulding of plastics utilizing hot runner systems. More particularly this invention relates to injection moulding nozzle tip assemblies and the configuration of such assemblies to minimize colour bleeding when changing from one coloured resin to another.
A conventional hot runner system for the injection moulding of plastics utilizes an injection nozzle or an array of such nozzles to force or xe2x80x9cinjectxe2x80x9d molten resin (xe2x80x9cmeltxe2x80x9d) into a void defined between core and cavity parts of a mould. Each nozzle is made up of a nozzle body or housing and a nozzle tip secured to a xe2x80x9ctip endxe2x80x9d of the housing either by threaded engagement or by a xe2x80x9cnozzle capxe2x80x9d. A conventional nozzle cap is a ring which fits over the nozzle tip, engages a flange at the base of the nozzle tip and threadedly engages the tip end of the nozzle housing. The housing has a first run of a melt passage extending axially along its length through which melt passes. The tip has a second run of the melt passage which registers with the first run and provides a continuous melt passage through the nozzle tip assembly. The second run terminates in one or more outlets through the tip depending on the flow pattern required for the mould arrangement being used.
The nozzle tip, or at least an outer end thereof may be received in a xe2x80x9cgatexe2x80x9d insertxe2x80x9d. The gate insert forms the xe2x80x9cgatexe2x80x9d portion of the mould, which is that portion through which melt enters the mould.
The resin must be maintained in its molten state as melt until the mould is filled. As the gate insert and its surrounding mould part represent a large heat sink, preferably direct contact between the nozzle tip, which is of metal, and the gate insert should be avoided. One way to avoid such direct contact is to provide a xe2x80x9cgapxe2x80x9d or xe2x80x9cvoidxe2x80x9d between the outer end of the nozzle tip and the gate insert.
Initially the gate would be filled with air but later would fill with resin which seeps in during moulding. A disadvantage to such an arrangement occurs during changeover from resin of a first colour to resin of another colour. The trapped resin continues to xe2x80x9cbleedxe2x80x9d into the new resin producing unacceptable streaking and requiring a lengthy changeover period and many wasted parts before changeover is fully established.
One solution to the problem of bleeding is to fill the void between the gate insert and the nozzle tip with an insert sometimes called a xe2x80x9cgate well insulatorxe2x80x9d. The gate well insulator is a machined insert of an insulating material, such as a ceramic, which occupies the void in which resin would otherwise collect. This approach however has its own disadvantages which include, an expensive base material, difficulty in matching the insulator to the gate inserts and the tips, cracking of the insulators after a short period of use and sticking of the insulators in the gate insert when the mould is opened for cleaning.
According to the present invention, a nozzle cap is provided which has a housing end that threadedly engages the nozzle housing to secure the nozzle tip to the nozzle housing. The remainder of the cap substantially fills the space between the nozzle tip and the gate insert while maintaining a small clearance between the cap and both the nozzle tip and the gate insert in the region surrounding an outlet end of the nozzle. The clearance prevents direct conductive heat transfer from the outlet end of the nozzle tip to the gate insert yet is sufficiently small enough to substantially prevent melt to flow past it.
More particularly, a nozzle cap is provided for securing an injection moulding nozzle tip to a nozzle housing which is receivable in a gate insert. The nozzle cap has a body mountable over the nozzle tip. The body has a housing end for threadedly engaging the nozzle housing to secure a base of the nozzle tip in the housing. The body further has a tip end extending about an outlet end of the nozzle tip and having a passage therethrough registering with the nozzle tip to pass melt flow from the nozzle tip. The tip end is dimensioned to provide a first clearance between the outlet of the nozzle tip and an interior of the body sufficient to provide heat insulation while substantially preventing melt flow therebetween. The tip end further is dimensioned to provide a second clearance between the tip end and the gate insert sufficient to provide heat insulation while substantially preventing melt flow therebetween. The housing has a medial portion between the housing end and the tip end which is dimensioned to contact the gate insert.
The first clearance is preferably no greater than 0.005 inches and the second clearance is preferably no greater than 0.015 inches.
More preferably the first clearance is 0.005 inches and the second clearance is 0.010 inches.
An outer face of the nozzle cap may be provided with inter-engagement contours adjacent the tip end for engaging solidified resin between the nozzle cap and the gate insert to assist in holding the solidified resin to the nozzle cap during separation of the nozzle cap from the gate insert during mould cleaning. The inter-engagement contours may be grooves extending into the outer face.
The nozzle cap may be made of metal, which may be H13 steel with a wear and corrosion resistant surface. The wear and corrosion resistant surface may be provided by chromium plating, nickel plating or ion nitriding.